Re: [Xen-devel] [PATCH v4 07/17] x86/hvm: add length to mmio check op

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.06.15 at 15:36, <Paul.Durrant@xxxxxxxxxx> wrote:
> I think that also allows me to simplfy the patch since I don't have to 
> modify the mmio_check op any more. I simply call it once for the first byte 
> of the access and, if it accepts, verify that it also accepts the last byte 
> of the access.

That's actually not (generally) okay: There could be a hole in the
middle. But as long as instructions don't do accesses wider than
a page, we're fine with that in practice I think. Or wait, no, in the
MSI-X this could not be okay: A 64-byte read to the 16 bytes
32 bytes away from a page boundary (and being the last entry
on one device's MSI-X table) would extend into another device's
MSI-X table on the next page. I.e. first and last bytes would be
okay to be accessed, but bytes 16...31 of the access wouldn't.
Of course the MSI-X read/write handlers don't currently permit
such wide accesses, but anyway...

Jan


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